This invention relates to an arrangement for automatic—passive—running gap control on a two or multi-stage turbine which comprises at least first and second rotors within an outer casing, with stator vanes arranged upstream, between and downstream of these rotors, respectively.
On the turbine of aircraft engines, the clearance between the blade tips of the rotor and the casing adjacent to it or another stationary component are desirably kept as small as possible to minimize performance and fuel losses and ensure high efficiency of the engine in all flight phases. However, this is problematic in that the rotating and stationary components are subject to different dynamic loads and to different thermal loads, in particular in the various flight phases such as take-off, acceleration, continuous operation or deceleration, with their expansion and contraction characteristics deviating accordingly.
The clearance (running gap, blade gap) between the rotating blade tips and the stationary casing parts adjacent to the blade tips must be large enough to prevent rubbing between the stationary and the rotating components as they expand under transient conditions. Under continuous operating conditions, this clearance will, however, grow to an extent that efficient use of the energy input is not ensured.
In order to keep the running gap as constant and as small as possible in all operating phases to effectively use the energy input while preventing the rotating blade tips of the rotor from contacting the adjacent, stationary area of the casing in the take-off phase, a great variety of solutions for running gap control has been presented.
The known “active” solutions for setting the size of the running gap comprise the supply of cold compressor air or hot combustion gases to the casing or to the liner segments (interlayers) connected to it, using their expansion or contraction to actively control the gap size or adapting the expansion behavior of the stator to the thermal and dynamic expansion behavior of the rotor in the various operating phases.
“Active” systems for running gap control are, however, disadvantageous in that they incur a loss of compressor work or a reduction of turbine efficiency, respectively. Moreover, adequate control of the gap width between blade tips and liner segments is not possible in all operating phases. Finally, active systems are expensive since they require valve and control devices.
In order to resolve the problems associated with active gap control, Specification GB 2061396 proposes an arrangement provided in the casing interior for automatic “passive” running gap control between the blade tips and the liner segments arranged on the inner side of the turbine casing of a single-stage turbine. In the case of this “passive” running gap control, the liner segments, which are arranged remotely above the tips of the rotor blades, are held at the outer platforms of the stator vanes of the turbine on one side and at the outer platforms of a subsequent stator vane on the other side, while the inner platforms of the stator vane segments on both sides are each connected to a ring element (expansion ring) whose reaction to a certain thermal load corresponds to the thermal behavior of the rotor. Thus, in the event of an expansion or a contraction of the rotor, the ring elements connected to the platforms will become larger or smaller to the same extent as the rotor, the moveably held stator vane segments will be shifted and the liner segments attached to the vane segments will be set relatively to the rotor and in correspondence with the degree of expansion and contraction of the rotor.
This design, which also comprises a special fixation of the stator vanes to enable their radial movement, ensures the formation of a constant running gap between blade tips and liner segments in all operating phases of the engine. The arrangement described in the above is, however, unsuitable for two or multi-stage turbines.
Based on the radial setting of the liner segments in accordance with the expansion and contraction behavior of the rotor known from Specification GB 2061396, the present invention, in a broad aspect, provides an arrangement for two or multi-stage turbines for passively setting a running gap that is constant in various operating phases.